Presently surface acoustic wave (SAW) filters are widely used in instruments of mobile communication, etc. because they have the advantages of small size, light weight, and no adjustment requirement, as well as other advantages. For mobile communication which requires characteristics, such as low propagation losses and high attenuation, SAW resonator filters are suitable.
A SAW resonator filter includes an input interdigital transducer (input IDT), an output interdigital transducer (output IDT), and a reflector. The SAW resonator filter uses resonance phenomena of surface acoustic waves confined in the transducer structure including the input IDT, the output IDT, and the reflector to have a passband of a frequency range which satisfies resonance conditions. A bandwidth of the SAW resonator filter is proportional to an electromechanical coupling coefficient (k.sup.2) of the piezoelectric substrate.
The SAW resonator filters using one resonance can only have narrow bandwidths. In order for a SAW resonator filter to have a wide bandwidth, it is effective to couple a plurality of resonances so that the SAW resonator filter has a wider bandwidth than that using one resonance.
For example, standing waves of acoustic surface waves confined by the IDTs and the reflector can have a wider bandwidth by coupling two resonances (so-called the zeroth mode resonance and the first mode resonance) than by the use of one resonance. A SAW resonator filter including three IDTs to couple the zero-order mode and the secondary mode resonances is also effective.
On the other hand, to realize a SAW resonator filter having high attenuation, it is effective to concatenate two electrode structure arrays.
FIG. 1 shows one example of the SAW resonator filter including concatenated two electrode structure arrays.
A first electrode structure array is disposed on a piezoelectric substrate. The first electrode structure comprises an input/output IDT 1 having N1 pairs of electrodes, and receipt IDTs 2, 2' disposed outside the input/output IDT 1 and having N2 pairs of electrodes which are substantially the same pitch as the input/output IDT and, and reflectors 3, 3' disposed outside the receipt IDTs 2, 2'. The input/output IDT 1 and the receipt IDTs 2, 2' are arranged with the adjacent electrodes spaced from each other by a gap L between the centers thereof.
A second electrode structure array is disposed on the piezoelectric substrate. The second electrode structure comprises an input/output IDT 11 having N1 pairs of electrodes and disposed on the piezoelectric substrate, receipt IDTs 12, 12' disposed outside the input/output IDT 11 and having N2 pairs of electrodes which are substantially the same pitch as the input/output IDT 11, and reflectors 13, 13' disposed outside the receipt IDTs 12, 12'. The input/output IDT 11, and the receipt IDTs 12, 12' are arranged with the adjacent electrodes spaced from each other by a gap L between the centers thereof.
The receipt IDTs 2, 2' of the first electrode structure array, and the receipt IDTs 12, 12' of the second electrode structure array are connected respectively by lines 4, 4' to concatenate the first and the second electrode structure arrays. The lines 4, 4' are connected with each other by a line 5 and have the same potential. The line 5 is provided as required.
Input signals are inputted to the input/output IDT 1 of the first electrode structure array in the first stage. Acoustic surface waves excited by the input/output IDT 1 are multi-reflected and received by the receipt IDTs 2, 2'. Energy of the received surface acoustic waves is converted to electric signals by the receipt IDTs 2, 2' and is supplied to the second electrode structure array on the second stage by the line 4, 4'.
In the second electrode structure array in the second stage, the surface acoustic waves are excited by the receipt IDTs 12, 12' and received by the input/output IDT 11. Energy of the received acoustic surface waves is converted to electric signals by the input/output IDT 11 and outputted.
The SAW resonator filter including the thus concatenated two electrode structure arrays is a filter circuit having the symmetrical structure with respect to the concatenation plane as a reflection plane. It is known that such a filter circuit is represented by a symmetrical lattice-type circuit having a serial arm impedance Za and a parallel arm impedance Zb, as shown in FIG. 2.
In the symmetrical lattice-type circuit of FIG. 2, when absolute values .vertline.Za.vertline., .vertline.Zb.vertline. of the serial arm impedance Za and the parallel arm impedance Zb are local minimum, the SAW resonator filter has resonance as one system. When absolute values .vertline.Za.vertline., .vertline.Zb.vertline. of the serial arm impedance Za and the parallel arm impedance Zb are local maximum, the SAW resonator filter has antiresonance as one system. Standing waves corresponding to the antiresonance are present in the SAW resonator filter.
Here, adjustment to match a resonance frequency of the serial arm impedance Za with an antiresonance frequency of the parallel arm impedance Zb and match a resonance frequency of the parallel arm impedance Zb with an antiresonance frequency of the serial arm impedance Za is made to obtain good filter characteristics.
Prior art relating to a SAW resonator filter which has a widened bandwidth as a result of such method is described below.
Japanese Patent Laid-Open Publication No. Hei 06-85605/1994 discloses a surface acoustic wave filter which is suitable to widen bandwidths. This surface acoustic wave filter is intended to realize wide bandwidths by using a two-IDT structure in a triple mode. When an outside-band spurious signal is high, this acoustic surface wave filter, which has the two-IDT structure, cannot effectively reduce the spurious signal by adjusting pair numbers of the IDTs.
Japanese Patent Laid-Open Publication No. Sho 64-82706/1989 discloses an acoustic surface wave filter having a narrow bandwidth and large attenuation outside the band. This acoustic surface wave filter matches a peak of internal reflection with a peak of reflected waves between digital electrodes to thereby obtain a double mode. This acoustic surface wave filter, which is a double-mode acoustic surface wave filter, has failed to realize sufficient propagation loss reduction and wide bands.
Japanese Patent Laid-Open Publication No. Hei 05-315886/1993 discloses an acoustic surface wave filter having small ripples and large outside-band attenuation. This acoustic surface wave filter is intended to secure small ripples and wide passband widths by using a piezoelectric substrate having an above 10% electromechanical coupling coefficient, and differing an interdigital electrode number of an input interdigital transducer and that of an output interdigital transducer. In Japanese Patent Laid-Open Publication No. Hei 05-315886/1993, a 41.degree.-rotated Y-cut X-propagating lithium niobate substrate having an above 10% electromechanical coupling coefficient is used, as the piezoelectric substrate. When an above 40.degree. and below 45.degree.-rotated X-cut Z-propagating lithium tetraborate substrate having a 1% electromechanical coupling coefficient, for example, is used as the piezoelectric, a specific passband width is only about 0.003, based on FIG. 9 of the specification of Japanese Patent Laid-Open Publication No. Hei 05-315886/1993, thus sufficiently wide bands cannot be realized.
Japanese Patent Laid-Open Publication No. Hei 05-267990/1993 discloses a surface acoustic wave filter having wide bandwidths and small propagation losses in high frequency regions. Japanese Patent Laid-Open Publication No. Hei 05-267990/1993 is intended to realize a surface acoustic wave filter of concatenated double mode surface acoustic wave filter having wide specific bands in high frequency regions. The acoustic surface wave filter, which is a double mode acoustic surface wave filter, has limitations regarding propagation loss reduction and widening bands.
Japanese Patent Laid-Open Publication No. Hei 07-38369/1995 discloses a multi-interdigital transducer-type acoustic surface wave filter having large passband widths and large attenuation amounts. Japanese Patent Laid-Open Publication No. Hei 07-38369/1995 is intended to realize an acoustic surface wave filter using three or more IDTs and defining a pitch between the IDTs. However, based on the study of Japanese Patent Laid-Open Publication No. Hei 07-38369/1995 by the inventors of the present application, the range defined by Japanese Patent Laid-Open Publication No. Hei 07-38369/1995 includes the double mode and modes larger than the double mode, and even a 4.ltoreq.n.ltoreq.6 range where preferable propagation characteristics are available includes the double mode and larger modes than the double mode. Accordingly band widening technique based on the operation mode is not disclosed.
Japanese Patent Laid-Open Publication No. Hei 01-231417/1989 and Japanese Patent Laid-Open Publication No. Hei 02-202710/1990 also disclose acoustic surface wave filters having wide passband characteristics by the use of the double mode resonance. These acoustic surface wave filters, which are double mode acoustic surface wave filters, have limitations regarding propagation loss reduction and band widening.
As described above, acoustic surface wave filters having low propagation losses and wide bandwidths have been conventionally intended, but no acoustic surface wave filter having a sufficiently large passband width and sufficiently large outside-band attenuation amounts has been realized.
On the other hand, the bandwidth of a SAW resonator filter depends on the electromechanical coupling coefficient (k.sup.2) of the piezoelectric substrate. The recent rapid development of the mobile communication requires SAW filters having bandwidths suitable for used bandwidths and low propagation losses. Piezoelectric substrates of quartz, LiNbO.sub.3, LiTaO.sub.3, or others are used depending on used bandwidths.
The piezoelectric substrates are usually classified in to those having large electromechanical coefficients k.sup.2 and large frequency deflections with respect to temperature, and in to those having small electromechanical coefficients k.sup.2 and small frequency deflections with respect to temperature.
The latter piezoelectric substrates having small electromechanical coefficients k.sup.2 and small frequency deflections with respect to temperature are suitable for narrow band-uses, such as IF filters, etc. and will have significant practical merits if they can have wide bands.
The former piezoelectric substrates having large electromechanical coefficients k.sup.2 and large frequency deflections with respect to temperature are suitable for applications requiring wide band characteristics, such as front ends, etc. and will have significant practical merits of increase of numbers of pass channels, etc. if they can have wider bands.
As described above, if the conventional acoustic surface wave filters can further widen bands, they will be able to find more applications while using merits of the respective piezoelectric substrates.
However, the conventional acoustic surface wave filters, especially the SAW resonator filters can not have bandwidths which are wide enough to enjoy the merits of the respective piezoelectric substrates.
An object of the present invention is to provide an acoustic surface wave filter whose band width is wider than that of the conventional acoustic surface wave filters, and in addition, has further improved outside-band spurious signal.